/Engine/Source/Runtime/PakFile/Public/IPlatformFilePak.h
Helper function to return Filename's relative path from the mount point. Returns null if Child is not equal to MountPoint and is not a child path under MountPoint, else returns pointer to the offset within Child after the MountPoint. If child equals MountPoint, returns null; The MountPoint itself is not a valid Filename, since Filenames must have non-zero length and are added on to the MountPoint.
static const TCHAR * GetRelativeFilePathFromMountPointer ( const [FString](API\Runtime\Core\Containers\FString) & Child, const [FString](API\Runtime\Core\Containers\FString) & MountPoint )